Studies and observations by scientists indicate that upstream fish migration at Khone Falls can be improved by careful design and management after the Don Sahong dam is built, a consultant has said.
Dr Tobias Coe of the UK company Fishtek Consulting told a technical workshop held in Vientiane recently that the developer plans to compensate for the blockage of the Sahong channel by improving other natural pathways for upstream migration and assisting the Lao government agencies to reduce fishing pressure in all channels around the Don Sahong Hydropower Project in the south of Laos.
Fishtek Consulting has been retained by the Don Sahong hydropower project's developer to advise on the upstream movement of fish across Khone Falls, a fault line that is a significant natural barrier to upstream fish migration from Cambodia to Laos.
Fishtek is also a consultant to the developer of the Xayaboury dam in northern Laos.
“Unlike the complex technology that is required for the multiple fish passage systems at Xayaboury, the Don Sahong project will rely on improvements to existing natural channels for fish migration,” Dr Coe said.
By studying the terrain of the Khone Falls area in both the wet and dry seasons, scientists have come up with recommendations for widening and deepening several channels, and in some cases, constructing bypasses around rock bars and other obstacles.
Dr Coe explained that the Sahong Channel, where the dam will be built, may well have been the main passage for upstream migrating fish in the late dry season in the past. That situation has been changed by the advent of large Chinese reservoirs on the upper Mekong, which store water in the wet season and release it during the dry season.
Hydrological studies have found that the minimum dry season flow of water in some important fish passage channels at Khone Falls is now up to five times greater than before the large storage dams in China were built.
Increased flow makes these channels more passable by fish, by increasing water depth and creating flow around and over natural obstacles.
The developer has also improved two alternative channels, by deepening them to increase flow, reducing slopes at key points and removing natural obstacles, so that fish can pass upstream more easily.
Don Sahong project staff are also working with Lao fisheries agencies and local fisherfolk to progressively remove large traps from channels that provide alternative fish migration routes.
An increase in the number of large fish traps in the Sadam Channel over recent years bears witness to the fact that the removal of natural and manmade obstacles from this channel, right next to the Sahong Channel, has in creased its attractiveness to migrating fish.
“The Sadam Channel will be an ideal fish passage, once these traps are removed,” Dr Coe said.
One strategy is to replicate and improve on conditions in the Sahong Channel in other channels, in order to improve fish migration.
“Careful design, combined with improved fisheries management has the potential to provide an overall positive impact on upstream fish migration, and this is the intention of the project,” Dr Coe told regional stakeholders and international hydropower experts at the Technical Workshop.
It will create a head pond that will be one of the smallest reservoirs of any hydropower project in the Mekong basin, he said.
The Lao government will proceed with development of the 260 MW Don Sahong run-of-river project, noting there is widespread support from those who understand that fish migration through the area will be better managed.
The National Assembly gave its stamp of approval to the project during its ordinary session in July.